In nature, individual ticks can be concomittently infected with more than one species of intracellular bacteria. Some may be important human pathogens acquired either from an infectious bloodmeal or transovarially. Others may be nonpathogenic symbionts acquired transovarially. The consequences and epidemiological significance of multiple bacterial infections in ticks are largely unexplored. We propose that the interactive effects between certain intracellular bacterial species within individual ticks may be of sufficient prevalence and magnitude to alter the vector competence of ticks. Ovaries of Dermacentor andersoni ticks harboring a particular nonpathogenic species of spotted fever group rickettsia, namely Rickettsia peacockii, have been shown to be refractory to subsequent co- infection with otherwise invasive R. rickettsii. Thus, ovarial infection of D. andersoni with R. peacockii rickettsiae interferes or blocks normal transovarial transmission of R. rickettsii. Infections with two or more closely-related rickettsial species may result either in interference (i.e., one species inhibits the development and/or transmission of the other), mutualism (i.e., enhanced transmission of both species), or phenotypic mixing (i.e., increased or decreased virulence). This proposal specifically addresses transovarial interference of the human pathogen, Rickettsia rickettsii, in the tick vector transovarially infected with nonpathogenic rickettsiae. We will investigate the phenomenon of transovarial interference in the context of its epidemiological significance and the mechanism(s) by which it occurs. Two specific aims are proposed: 1) We will investigate the prevalence and relative efficiency of transovarial interference of R. rickettsii as medicated by other indigenous species of nonpathogenic spotted fever group rickettsiae within D. andersoni and D. variabilis. This will entail field collections of ticks and laboratory evaluation of the interference potential of these agents; and 2) We will initiate studies to examine the molecular mechanism (s) of transovarial interference of R. rickettsii by R. peacockii and other nonpathogenic spotted fever group rickettsiae.